As is known, there are many designs of aircraft, herein referred to as convertiplanes, seeking to combine the advantages of fixed-wing propeller aircraft and of helicopters, for example by use of tilt-rotors or by tilt-wings. Of these, arguably only the twin tilt-rotor designs are close to full production.
Twin tilt-rotor designs have a long history. In 1934, U.S. Pat. No. 1,951,817 was granted to Blount for his “airplane-helicopter”. In 1958 the Bell XV-3 became the first tilt-rotor to convert to the aeroplane mode; a 15-year test programme proved that it could fly safely and smoothly throughout its flight envelope. The XV-3 was limited in speed by its use of blades designed for helicopter flight, and this was remedied in Bell's next project, the XV-15, by use of high twist blades. The first XV-15 was rolled out in 1976, and by 1986 the two prototypes had achieved an unofficial record for rotorcraft of 301 knots, had accumulated 530 flight hours and made 1,500 conversions. In that year a scaled-up version of the XV-15, a joint proposal by Bell and Boeing called the V-22 Osprey, was approved for full-scale development for the US armed forces, achieving its first flight in 1989. The V-22 eventually entered low-rate initial production in 1999, but subsequently was grounded after two crashes in 2000; revised flight tests began in May 2002 and it is now in limited production.
The most recent twin tilt-rotor is the Bell Agusta BA 609, which made its maiden flight on 7 Mar. 2003 as the world's first civil tilt-rotor and is configured generally as the V-22.
This history from 1934 to the present day shows painfully slow progress for the twin tilt-rotor concept. Over the same timescale a wide variety of helicopter designs have achieved large-scale production and valuable service worldwide. But even that success for helicopters is modest compared to that of fixed wing aircraft which have achieved production quantities, utilisation, safety, reliability, speed, range and altitude performance that are far better and at much lower cost than helicopters of equivalent payload.
The reasons for the relative success (or lack of it) of these three concepts lie in the economic cost of acquisition and operation.
Firstly, comparing a propeller driven fixed wing design with a helicopter design of equal payload, fuel load and installed turbine power:                The weight of high cost machinery, ie. powerplant, transmission, rotor or prop blading etc needed by the fixed wing design is half that of the helicopter.        The fixed wing design would be expected to have twice the cruise speed and much greater range.        But the helicopter is VTOL capable, whereas the fixed wing design is not.        
Secondly, comparing a twin tilt-rotor design with the above two designs:                The twin tilt-rotor can be both CTOL and VTOL capable, the other two designs cannot.        The range of the twin tilt-rotor will be much better than the helicopter but significantly less than the fixed wing design.        However, in order to match the speed and CTOL capability of the fixed wing design and to match the VTOL capability of the helicopter, the twin tilt-rotor needs twice the installed turbine power.        The weight of high cost machinery, ie powerplant, transmission, rotor or prop is twice that of the helicopter and four times that of the fixed wing design.        
It is an object of at least the preferred embodiments of the present invention to provide a tilt-rotor aircraft in which the above disadvantages are reduced.
The term “rotor” is to be construed broadly, to include not only an open (helicopter-type) rotor, but also a ducted fan or ducted rotor.
In another aspect, the invention provides a tilt-rotor aircraft comprising a fuselage, wings for sustained forward flight, and at least one rotor tiltable between a position providing lift and a position providing propulsion for forward flight, the rotor or rotors being carried by supporting structure mounted on the fuselage and being disposed on or symmetrically about the longitudinal centre line of the aircraft.
In another aspect, the invention provides a tilt-rotor aircraft comprising a plurality of rotors carried by at least one tiltable nacelle on the longitudinal centre line of the aircraft.
The, or each, nacelle may have a pair of contra-rotating rotors which are coaxial, or on parallel axes, or intermesh.
A said nacelle or supporting structure may be mounted to pivot and optionally also translate about an axis extending transversely of an upper part of the aircraft fuselage.
The nacelle may contain at least one engine. Alternatively the engine(s) may be located elsewhere in the aircraft, and power may be delivered mechanically to the rotors or via a local power turbine, electric motor, hydraulic motor or other transmission.
The rotors preferably are driven by a plurality of engines via a transmission such that all the rotors continue to be driven if an engine fails.
The nacelle may be mounted to pivot and optionally also translate about an axis extending transversely of an upper part of the fuselage.
At least inboard portions of wings of the aircraft maybe moveable so as to present leading edges to the airflow generated from the rotors in lift mode.
In another aspect the invention provides a tilt-rotor aircraft comprising a tiltable rotor assembly on the longitudinal centre line of the aircraft moveable between a lift mode and a forward flight mode, inboard portions of the wings of the aircraft being moveable so as to present leading edges to the airflow generated by the rotors in lift mode.
A said moveable portion may be rotatable and/or translatable transversely or longitudinally about a fixed beam projecting from the fuselage of the aircraft.
The beam may extend to a fixed outboard portion of the wing. The fixed beam may be offset from the axis of tilt of the rotors either forwardly or rearwardly or vertically.
Preferably each wing may have at least two substantially parallel moveable portions.
A said moveable portion may be configured to act as a control surface when the aircraft is in lift mode and/or in transition between lift and forward flight modes, the aircraft also comprising control means for operating the control surface.
The underside of the aircraft fuselage may be shaped to reduce download forces on the fuselage from the airflow generated by the rotor or rotors in lift mode.
Preferably there is a control surface on the fuselage, operative when the aircraft is in a lift mode and/or in transition between lift and forward flight modes.
In a further aspect the invention provides a tilt-rotor aircraft comprising a tiltable rotor assembly moveable between a lift position and a forward flight position in front of or behind the fuselage.
Preferably in this aspect the aircraft is of a twin-boom layout, wherein booms extend rearwardly from the wings of the aircraft to support the aircraft's empennage, the rotor assembly being behind the fuselage and disposed between the booms when the aircraft is in forward flight mode.
In one embodiment, the rotor assembly is below the fuselage when in the lift position. In another embodiment it is above the fuselage.
The preferred embodiments of the invention have all the powerplant, transmission and rotor components within a single rotornacelle, allowing some components such as cross-wing transmissions to be eliminated and others such as support structures to be simplified.
In another aspect this invention provides a tilt rotor aircraft comprising a fuselage, wings for sustained forward flight, and a plurality of rotors, each rotor being independently and sequentially tiltable between a position proving lift and a position proving propulsion for forward flights.
In a preferred embodiment the powerplant may be within the fuselage.
Preferably the rotors are mounted on, or symmetrically about, the centre-line of the aircraft.
The rotors may be mounted at an angle of inclination to the roll axis or to the pitch axis of the aircraft. The angle of inclination may be up to 45 degrees, for example between 12½ and 32½ degrees, and preferably being substantially twenty two and a half degrees.
The angle of inclination may be variable.
Preferably the aircraft may comprise rotors which are substantially in mesh.
The aircraft may comprise a mechanism for varying the relative phase of the rotors to permit sequential tilting, preferably substantially in mesh.
Preferably the mechanism may comprise a cross-shaft.
The tilt axis of the rotors may be vertically or longitudinally offset from the point of convergence of a longitudinal axis of each rotor.
The rotors may have a common cross-shaft. The gear ratios between the cross-shaft and the rotor shaft for a two bladed rotor design may be of a 2:3 ratio. For a three or more bladed rotor design the cross-shaft preferably includes a differential gearbox with means to input mesh phase correction.
The flight positions are preferably to the rear of the fuselage and the lift positions are preferably substantially vertical, or tilted towards the front of the fuselage by up to ten degrees, preferably up to five degrees. The flight positions may be between ninety and one hundred degrees from the lift positions. Preferably the flight positions are substantially ninety five degrees from the lift positions.
In a further aspect the invention provides a tilt-rotor aircraft comprising a fuselage, wings for sustained forward flight, front and rear rotors mounted on the aircraft fuselage for providing lift at least one of the rotors being tiltable between a lift position and a position providing propulsion for forward flight.
Both rotors may be tiltable between lift and propulsion positions.
One rotor may be deployable to an autogyro configuration for forward flight, or can be folded in flight.
The aircraft may be operable in three flight modes, a first of which is in use below a first air speed, a second of which is in use between the first airspeed and a second airspeed, and a third of which is in use above the second airspeed. The aircraft may comprise a controller which can automatically manage transitions between these flight modes.
Alternatively the aircraft may comprise means for sensing dynamic air pressure arising from air speed, the aircraft being operable in three flight modes determined with reference to dynamic pressure, the first-mode being used below a first dynamic pressure, the second mode between the first dynamic pressure and a second dynamic pressure and the third mode above the second dynamic pressure.
The second flight mode is preferably a compound mode in which at least one of the rotors is in the flight position or at least one of the rotors is in the lift position. Preferably at least one of the rotors is substantially in the flight position and one of the rotors is substantially in the lift position in the second flight mode.
The first air speed or first dynamic pressure may be sufficient for the wings to take substantially half of the aircraft lift, and the second air speed or second dynamic pressure may be sufficient for the wings to take substantially all of the aircraft lift.
The rotors may be moveable into a feathered position so that they may be tilted between lift and flight positions or held in or between those positions more easily.
Any of the aspects of the invention described above may be combined with any other of the aspects of the invention to provide an improved tilt-rotor aircraft.
Other advantages which may be achieved with various embodiments are the following:—                Co-axial, contra-rotating rotors enable gyroscopic forces and rotor torques to be balanced within the rotornacelle or at the fuselage rather than across the aircraft wing structure.        An aircraft having a single centrally mounted and aerodynamically symmetrical rotornacelle, is much less vulnerable to asymmetric airflow and asymmetric thrust. This is a particular concern in low speed and hovering flight. For example a twin-prop airplane that loses all thrust from one side becomes more difficult to manoeuvre or land or take off at low speed, where yaw problems can escalate into irrecoverable roll. Wing mounted twin tilt-rotor designs typically guard against this problem of asymmetric loss of power by the use of complex cross-wing transmissions between their engines, however this cannot compensate for major asymmetry of airflow from the rotor on one wing to the other. Such problems occur when one rotor enters or leaves the vortex ring state before the other, and when in ground effect the flow symmetry is destroyed by proximity to other aircraft disturbing the air or by physical discontinuities of the effective ground surface.        The aircraft wing design can be optimised without the restraints imposed by mounting engines, nacelles or rotors on the wings.        The aircraft can be designed for efficient lift in both transition and helicopter modes by minimising the download from the rotor airflow that acts on the fuselage and wing. This is achieved by ventral fairing of the fuselage, and by aligning the inboard portions of the wing to the airflow. Both methods also provide the possibility of use as control surfaces for the aircraft.        In “pusher” embodiments the field of view from the cockpit is unobstructed by the tilt rotors.        